MEMS accelerometer devices have low production costs, good device matching performance, and manufacturing processes compatible with IC process technology, so that MEMS accelerometer devices have been widely studied. The interdigital capacitive sensors with high sensitivity, good linearity, and low noise are the most commonly used MEMS accelerometer devices.
Although three-axis acceleration sensors are the research focus of MEMS accelerometers, however, single-axis acceleration sensors still play a decisive role, because single-axis accelerator technology is easy to design, and can have simple processes and stable performance. For example, an acceleration sensor for automatic releasing of airbags is a type of single-axis acceleration sensor, which should have a high directional sensitivity, high acceleration range (e.g., the range of airbag acceleration sensors is at least above 50 g).
In order to avoid false activation of an airbag, an important feature of such acceleration sensor is that it is not sensitive to the influence of other directional accelerations. For example, the measurement values of the X-axis acceleration should not be significantly deviated due to the acceleration in the Y-axis or Z-axis.
FIG. 1 is a top plan view of a structure of a uniaxial airbag acceleration sensor, as known in the prior art. The acceleration sensor includes a rectangular mass bar 101, a frame-shaped spring 102 disposed on opposite short sides of rectangular mass bar 101 for securing the rectangular mass bar, an interdigital structure 103 disposed on each long side of rectangular mass bar 101, and an interdigital electrode 1031 corresponding to interdigital structure 103. This structure of an acceleration sensor is vulnerable to cross effects of acceleration in other directions and tends to cause false activation of the airbag.
The conventional design of an MEMS acceleration sensor may be mechanically modeled using the following simulation data: mass 101 has a thickness of 50 μm, a length of 1000 μm, a width of 200 μm. Spring 102 disposed on both narrow sides of mass 101 is configured to secure the mass bar and has a width of 7 μm. Mass 101 has an elastic modulus of 1.69×1011 Pa, a Poisson's ratio (PRXY) of 2.3, a density (DENS) of 1.4×103 kg/m3. For simplicity, the interdigital structure disposed along the long sides of mass bar 101 is not included in the simulation because its mass is negligible and does not affect the movement of mass bar 101.
In the case where the accelerations in the X- and Y-axes are 30 g, simulation data of the mechanic model is as follows: the displacement of mass bar 101 in the X-axis is 0.119 μm, and the displacement of mass bar 101 in the X-axis is 0.114 μm. Obviously, such large displacement in the Y-axis will adversely affect the acceleration detection of the MEMS acceleration device in the direction of the X-axis.
Thus, the structure of a conventional single-axis acceleration sensor is susceptible to effects of accelerations in other directions (e.g., the Y-axis direction) other than the predetermined direction (e.g., the X-axis direction), and the acceleration detection of the acceleration sensor will be negatively affected, ultimately causing an incorrect operation of the acceleration sensor control device (e.g., an airbag indicator lamp). Thus, there is a need to develop a new structure for the acceleration sensor to solve the above-described problems.